The present invention relates, in general, to schemes for controlling display devices, and more particularly to a novel scheme for controlling drive signals for display devices.
Field emission devices (FEDs) are well known in the art and are commonly employed for a broad range of applications including image display devices. An example of a field emission device (FED) is described in U.S. Pat. No. 5,191,217 issued to Kane et al. on Mar. 2, 1993. One prior method of controlling such FEDs, commonly referred to as pulse width modulation, utilizes a digital video word to encode the intensity of an image that is to be displayed by the FED during a particular display time. The value of the digital word represents a portion of the total display time that a fixed drive voltage is applied to the FED or the FED active time. One problem with such prior control methods is the resolution that can be obtained. Because the FED appears to a drive circuit as a large capacitor, the drive signal has a large rise time and fall time. Consequently, the rise time and fall time can represent a large portion of the FED active time. For low intensity signals, the rise time and the fall time may be greater than the total FED active time. For example, for an eight-bit video word the minimum display time may be ten nanoseconds which typically is less than the rise time required to drive a FED. Consequently, no image would be displayed.
Another method, commonly referred to as amplitude modulation, varies the voltage value applied to each pixel to control the intensity. Because of the resulting low drive voltage increments, the method is susceptible to noise which results in a loss of display quality.
Accordingly, it is desirable to have an FED control method that has a minimum FED active time which is greater than the minimum time increments represented by the digital video word, that does not have a single or fixed drive signal, that has a minimum time increment that is greater than the rise and fall time of the drive signal of the FED, and that maximizes the minimum voltage drive increments applied to the FED.